In electrodeless gas discharge lamps that operate according to the induction principle, an electric discharge or a plasma is generated and maintained in a discharge vessel or lamp envelope by means of a high frequency alternating electromagnetic field. The transformation of electric energy into light is achieved by the excitation of atoms in the plasma discharge by means of impact ionization in the electric field. In contrast to the widely used fluorescent lamps which mainly use hot electrodes (HCFL) or, less commonly, cold electrodes (CCFL), electrodeless gas discharge lamps do not need any electrodes at all. The electric excitation field that the discharge triggers and feeds is generated by an oscillating high frequency magnetic field. It is well known that the absence of electrodes in the discharge vessel makes it possible to prolong the useful service life of the gas discharge lamps by five to ten times. Familiar ageing mechanisms for gas discharge lamps due to evaporation or electric erosion (sputter processes) of the electrode coating do not occur in electrodeless lamps. And by the very nature of the electrodeless lamps, there are no electrode losses, so that the efficiency of electrodeless gas discharge lamps is greater than that of HCFL and CCFL. Since there are no electrodes within the discharge vessels and thus no electrode chemistry that need be taken into account, the choice of possible active media for the purpose of generating the discharge plasma within the discharge vessel is made very much wider. Whereas nowadays mixtures of metal vapor, particularly mercury vapor, and rare gas are commonly used as active media, in the case of electrodeless lamps non-toxic, mercury-free active media may also come into consideration.
In the prior art, two different types of electrodeless gas discharge lamps which operate on the principle of magnetic induction are basically known. Commercially available at the present time are the electrodeless gas discharge lamps made by Philips and Matsushita, which use rod-shaped cores that extend into the lamp envelope, and also the lamps from Osram and Hongyuan, which use annular discharge tubes onto which the toroidal ferrite cores are mounted. For the sake of completeness, it should be mentioned that electrodeless gas discharge lamps are also known that operate without magnetic cores, a coil being wound directly about the glass envelope.
DE 30 08 535 C2 by Philips describes an electrodeless gas discharge lamp having a lamp base and a lamp vessel filled with a metal vapor and rare gas in which a multi-part annular core made of magnetic material, fed by a high frequency oscillator disposed in the lamp base, is so disposed that it partly extends through a tubular channel within the lamp vessel. The magnetic core consists of two separable parts, of which one part lies within the tubular channel of the lamp vessel and the other part is located outside the lamp vessel in the base. The magnetic core outside the lamp vessel carries an induction coil that is fed by the high frequency oscillator. Further windings of a copper foil strip are wound about the part of the toroidal core that lies in the tubular channel within the lamp vessel to facilitate ignition of the lamp.
DE 100 58 852 A1 describes an electrodeless low pressure gas discharge lamp having a ball-shaped, ring-shaped, pear-shaped or ellipsoidal glass body which is used as a gas discharge receptacle. The electric energy is introduced into the discharge receptacle in an inductive manner using a ring-shaped closed ferrite core which is partially located within the discharge receptacle and is provided with a primary winding that is fed in a frequency range of 100 kHz to 500 kHz. Part of the ring-shaped ferrite core is introduced into the discharge receptacle by means of a vacuum-tight passage that is inserted into the glass body. The part of the ferrite core having the primary winding is disposed in a lamp base outside the glass envelope.
DE 28 09 957 describes a fluorescent lamp having a substantially globular envelope that contains a gaseous medium and has a channel. An annular magnetic core partly extends through this channel and carries a winding to induce an electric field in the gaseous medium.
Available on the market under the name Osram Endura® is an electrodeless gas discharge lamp made by Osram GmbH which comprises an annular tubular discharge envelope on opposite sides of which two toroidal cores that carry exciter windings are mounted. The gas discharge lamp operates like a transformer, the exciter windings forming the primary windings of the transformer and the gas discharge tube forming the secondary winding of the transformer into which electric power is inductively coupled.
All electrodeless gas discharge lamps of the prior art have the disadvantage that they generate an extensive amount of electromagnetic interference.
It is thus an object of the invention to provide an electrodeless gas discharge lamp whose properties with regard to electromagnetic interference (EMI/EMC) are an improvement on those of gas discharge lamps of the prior art.